Method and apparatus for detecting pattern defects

ABSTRACT

A pattern defect inspection apparatus having an optical system that provides uniform illumination not affected by uneven thickness of a thin film formed on a sample even when using monochromatic light and is capable of detecting fine defects with high sensitivity. The apparatus comprises a laser to illuminate a sample, a coherence suppression optical system to reduce laser beam coherence, a condenser means to condense the laser beam onto a pupil plane of an objective lens, a detector means to detect the light reflected from a circuit pattern formed on a sample (semiconductor substrate), and a measurement section to find defects from the acquired and focused image. Adverse effects from interference caused by uneven film thickness on the sample are reduced by changing the laser beam incident angle on the objective lens pupil plane so as to adjust the laser beam incident angle on the sample.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to pattern defect inspectionmethods and apparatus using a laser beam as illumination light, mainlyfor inspecting and observing micro pattern defects or foreign mattercontamination occurring in manufacturing processes for semiconductordevices and flat panel displays.

[0002] Circuit patterns tend to continually become finer and smaller assemiconductor devices become more highly integrated. Smaller and finercircuit patterns have spurred demand for higher resolution wheninspecting defects of circuit patterns formed on semiconductor wafers byphotolithographic processes using photomasks or reticles. One techniquefor enhancing resolution when detecting pattern defects is usingillumination light on shorter wavelengths from visible light toultraviolet light. Mercury lamps and xenon lamps, for example, were usedin the related art as illumination light sources. Only the requiredwavelengths are optically selected and utilized from among the variousline spectra emitted from these lamps.

[0003] Illumination from a light source lamp, however, contains only afew line spectra in the ultraviolet region. A larger size lamp withhigher power must therefore be used to obtain sufficient light intensitycausing the problem of lower lighting efficiency. Yet another problem isthat correcting the chromatic aberration of optical systems used forpattern inspection is difficult due to the wide spectral bandwidth.

[0004] Optical aligners used in semiconductor device manufacturing alsorequire high resolution. For this reason, optical aligners equipped witha krypton fluoride (KrF) excimer laser that emits light at a 248 nmwavelength are mainly used. Optical aligners using an argon fluoride(ArF) excimer laser that emits an even shorter 196 nm (193 nm?)wavelength have also been developed.

[0005] However, these excimer lasers are large in size and use fluorinegases that are harmful to the human body, so safety measures must beimplemented.

[0006] Recently, a great deal of attention is being focused onsolid-state YAG lasers as another type of ultraviolet laser. YAG lasersare capable of generating a third harmonic (355 nm wavelength) or fourthharmonic (266 nm wavelength) by wavelength conversion when the laserbeam is passed through a nonlinear optical crystal. This has lead todevelopment of compact, easy to handle ultraviolet lasers. These compactand easy to use ultraviolet lasers are highly effective in patterninspection apparatus.

[0007] Laser beams have superior coherence, but this causes enhancementand attenuation in the light flux when used to illuminate a sample andproduces an interference fringe on the sample. In a pattern inspectionapparatus using a laser, as disclosed in Japanese Patent JP-A No.271213/1999, a light beam emitted from a laser light source is guidedinto a fly-eye lens (micro-lens array) to form a multi-spot lightsource. This multi-spot light source is focused to strike a sample undertest so that the sample is illuminated with uniform light. The intensityof the light reflecting from the sample is then detected with a chargeintegration type CCD line sensor.

[0008] The aforesaid pattern defect inspection apparatus of the relatedart using a laser has the following problems.

[0009] A light beam emitted from a laser is transformed into amulti-spot light source by a fly-eye lens and focused by a condenserlens to illuminate the entire area of the sample under test. Theincident angle of the illumination light versus the sample under test isdetermined by the focal positions of the fly-eye lens and the condenserlens. When a thin film is formed on the surface of the sample, thereflected light from the sample contains light reflecting from thesurface of the thin film and also light reflecting from the lower layersurface of the thin film after penetrating into the thin film. The phaseof the light reflecting from the lower layer surface of the thin filmchanges on the surface of the thin film according to the thickness ofthe thin film, so that the reflected light intensity to be detected onthe surface of the sensor will vary.

[0010] Now we will discuss how the intensity of reflected light changesin cases where a thin film such as an insulating film is formed on thesurface of sample 1. A typical interference model is shown in FIG. 6.Here, the wavelength of illumination light 37 is set as ?, the incidentangle of illumination light 37 versus the normal line direction on thesurface of the sample is ?, the refractive index of an air layer 34 isn0, the thickness and refractive index of a thin film 35 are t1 and n1,and the refractive index of a semiconductor substrate 36 is n2. If theintensity of reflected light 38 on the surface of the thin film 35 isset as r01, and the intensity of reflected light 39 from the substrate36 after passing through the thin film 35 is r12, then the compositereflected light can be defined as R. These factors can be theoreticallymodeled as Fresnel equations and expressed by equations 1 to 4. Anexample of the calculated results is shown in FIG. 7. The horizontalaxis represents the thickness of the thin film 35 and the vertical axisthe composite light intensity R. Changes in the composite lightintensity versus the film thickness are plotted as shown in waveform 40.$\begin{matrix}{X = {\frac{4\pi \quad {n1t1}}{\lambda}\cos \quad \theta}} & \left( {{Eq}.\quad 1} \right) \\{{r01} = \frac{{n1} - {n0}}{{n1} + {n0}}} & \left( {{Eq}.\quad 2} \right) \\{{r12} = \frac{{n2} - {n1}}{{n2} + {n1}}} & \left( {{Eq}.\quad 3} \right) \\{R = \frac{{r01}^{2} + {r12}^{2} + {2{{r01r12cos}(X)}}}{1 + {{r01}^{2}{r12}^{2}2{{r01r12cos}(X)}}}} & \left( {{Eq}.\quad 4} \right)\end{matrix}$

[0011]FIG. 8 shows a cross section of the sample 1 on which circuitpatterns are formed. A circuit 41 and a circuit 42 are formed on asemiconductor substrate 36, and the entire surface of the sample 1 iscovered with an insulating film 35. Assuming, for example, that thecircuit 41 has a light density pattern while the circuit 42 has a highdensity pattern and also that the thickness of the insulating film 35 isnot uniform due to some reason, the thickness of the insulating film 35will be t10 on the circuit 41 and t11 on the circuit 42. As mentionedabove, if the thickness of the insulating film 35 varies, then thereflected light from the sample, which contains light reflecting fromthe thin film surface and light reflecting from the thin film lowerlayer surface (after penetrating into the thin film), changesaccordingly. FIG. 9 shows this change in the reflected light intensitycaused by the example of FIG. 8. The difference in reflected lightintensity between the thickness t10 and thickness t11 of the insulatingfilm 35 corresponds to a portion 44 on a waveform 43 that indicates therelation between the reflected light intensity and the film thickness. Achange (Rt1) can be observed in the reflected light intensity. When thepattern of the sample 1 is inspected under this condition, the change inthe reflected light intensity caused by the difference in thetransparent film thickness is detected as a change in brightness.

[0012] In inspection methods of the related art, a sample is illuminatedwith light incident on the sample at a certain angle. So, when thethickness of a transparent film formed over the surface of the patternvaries different positions, the reflected light intensity from thesample, which contains light reflecting from the surface of thetransparent film and also reflecting from the lower layer surface of thetransparent film, changes according to the position on the film (aninterference fringe pattern occurs). Due to this interference fringepattern, the reflected light intensity to be detected on a CCD linesensor varies according to the position on the thin film. To reduceadverse effects from uneven brightness or shading caused by theinterference fringe pattern, the CCD line sensor must be adjusted so asto detect dark areas and as a result, the detection sensitivity is alower level.

SUMMARY OF THE INVENTION

[0013] The present invention has the object of providing an opticalsystem to uniformly illuminate a sample without being affected byvariations in the thickness of the transparent thin film formed over thesurface of the sample even when using a monochromatic light source suchas a laser, as well as providing a highly reliable pattern defectinspection method and apparatus that ensure highly accurate inspectionwithout lowering detection efficiency.

[0014] In the pattern defect inspection method of the present invention,a laser beam emitted from a laser light source annularly scans on thepupil plane of an objective lens. This annularly scanning laser beam isirradiated onto a sample (pattern formed on a semiconductor substrateand covered with an optically transparent thin film). An optical imageof the sample produced by laser irradiation is acquired with an imagesensor and then processed to find defects in the pattern. In thispattern inspection process, the annular scan diameter of the laser beamis determined based on the thickness of the optically transparent thinfilm.

[0015] In the present invention, when a pattern formed on asemiconductor substrate is inspected through an optically transparentthin film coated over the pattern, a laser beam emitted from a laserlight source annularly scans on the pupil plane of an objective lens andthen illuminates the pattern on the semiconductor substrate placed on atable continuously moving along one direction. An optical image of thepattern thus illuminated is then acquired in synchronization with theannular scan of laser beam. Defects in the pattern can be detected byprocessing this image.

[0016] These and other objects, features and advantages of the inventionwill be apparent from the following detailed description of preferredembodiments of the invention as illustrated in the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a schematic diagram showing the pattern defectinspection apparatus of the first embodiment of the present invention;

[0018]FIG. 2 shows an embodiment of the light illuminating opticalsystem shown in FIG. 1, including coherence suppression optics;

[0019]FIG. 3 shows the laser beam scanning on the pupil of an objectivelens;

[0020]FIG. 4 shows how light flux enters the pupil of an objective lens;

[0021]FIG. 5 shows waveforms used for directing the coherencesuppression optics shown in FIG. 2;

[0022]FIG. 6 is a cross section of a semiconductor substrate showingthin film interference;

[0023]FIG. 7 shows changes in the reflected light intensity due to thinfilm interference shown in FIG. 6;

[0024]FIG. 8 is a cross section of a sample on which circuit patternsare formed;

[0025]FIG. 9 shows thin film interference caused by the circuit patternsshown in FIG. 8;

[0026]FIG. 10 shows thin film interference found in the presentinvention;

[0027]FIG. 11A shows the incident angle of a light beam entering nearthe edge of the objective lens pupil;

[0028]FIG. 11B shows the incident angle of a light beam entering nearthe center of the objective lens pupil;

[0029]FIG. 12 shows changes in reflected light intensity when a lightbeam strikes the sample at different incident angles;

[0030]FIG. 13A is a schematic drawing showing inspection resultsobtained without optimizing the incident light angle;

[0031]FIG. 13B is a schematic drawing showing inspection resultsobtained with the present invention by optimizing the incident lightangle;

[0032]FIG. 14 is a pictorial drawing of the light illuminating opticalsystem including coherence suppression optics, implemented as the secondembodiment of the present invention;

[0033]FIG. 15 show a light flux focused on the pupil of the objectivelens after having passed through the optics shown in FIG. 14;

[0034]FIG. 16 is a pictorial drawing of the light illuminating opticalsystem including coherence suppression optics, implemented as the thirdembodiment of the present invention;

[0035]FIG. 17A is a front view of the mirror shown in FIG. 16;

[0036]FIG. 17B is a side view of the mirror shown in FIG. 16;

[0037]FIG. 18 shows a movement track of the mirror shown in FIG. 17;

[0038]FIG. 19 shows a cross section of the mirror shown in FIG. 17;

[0039]FIG. 20A is a schematic diagram showing a front view of the TDI(time delay integration) image sensor;

[0040]FIG. 20B is a side view of the TDI (time delay integration) imagesensor;

[0041]FIG. 21 is a block diagram of comparator 18 shown in FIG. 1;

[0042]FIG. 22 is a schematic diagram showing the pattern defectinspection apparatus of the second embodiment of the present invention;

[0043]FIG. 23 shows a flowchart for process control using patterninspection of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0044] Preferred embodiments of a high-resolution optical system and apattern defect inspection apparatus of the invention are next describedin detail using the accompanying drawings of FIG. 1 to FIG. 23.

[0045]FIG. 1 is a schematic diagram showing the pattern defectinspection apparatus of the first embodiment of the invention. Referencenumeral 1 shown in FIG. 1 denotes a sample constituted for example by asemiconductor wafer (device under test) on which is fabricated a circuitpattern to be inspected. The sample 1 is placed and held on a Z stage 2(by means for example of a vacuum chuck or electrostatic chuck not shownin drawing) that moves in the Z direction and rotates. AY stage 4 and anX stage 3 can be independently moved to any desired position undercontrol from a stage control circuit 100. The position of each stage isconstantly detected with a length measuring device or position sensor(not shown in drawing). The detected position data on the X stage 3 andY stage 4 is input to a central processing unit (CPU) 19. The stagecontrol circuit 100 is connected to the central processing unit 19. Theinvention employs an ultraviolet laser light source (ultraviolet lasergenerator) 5 that emits a far ultraviolet laser beam to illuminate thesample with far ultraviolet light of high intensity. A laser beam L1emitted from the ultraviolet laser light source 5 is guided into anobjective lens 11 by way of a beam expander 7, coherence suppressionoptics 8, lens 9, and beam splitter 10, and then illuminates the sample1. The beam expander 7 enlarges the ultraviolet laser beam to a certaindiameter. The enlarged laser beam L1 is condensed by the lens 9 onto aposition near the pupil 12 of the objective lens 11 and then illuminatesthe sample 1.

[0046] The light reflecting from the sample 1 when illuminated with thelaser beam L1 is focused onto the photosensitive surface of an imagesensor 14 by way of the objective lens 11, beam splitter 10, andfocusing lens 13 which are perpendicularly installed above the sample 1,and an optical image of the sample 1 is thus obtained. The image sensor14 can, for example, be a charge integration type sensor (time delayintegration type image sensor: abbreviated to TDI sensor) capable ofdetecting ultraviolet light, which outputs a grayscale image signalaccording to the brightness (gray level) of the light reflecting fromthe pattern formed on the sample 1 under test.

[0047] The TDI sensor 14 is made up of an array of linear image sensorsconnected in a multistage configuration as shown in FIG. 20A and FIG.20B. The sample 1 is first viewed with the first stage linear imagesensor 14, in synchronization with movement of the Y stage thatcontinuously moves by a control signal from the central processing unit19. The signal obtained here is transferred to the second stage linearimage sensor 14 ₂. Next, when the area on the sample 1 whose image wasacquired with the first stage linear image sensor 14 ₁ moves to theposition of the second stage linear image sensor along with movement ofthe Y stage, the image in that area is again acquired with the secondstage linear image sensor 14 ₂ and the detected signal here is added tothe signal already transferred from the first stage linear image sensor14 ₁. By repeating this process on all of subsequent stage linear imagesensors up to the last stage image sensor 14 _(n), the signals detectedby each linear image sensor stage are all accumulated and output.

[0048] In the above configuration, the central processing unit 19 issuesan instruction to the stage control circuit 100 to drive the stage 3, sothat the sample 1 moves at a constant speed along one direction. At thesame time, an optical image of the pattern fabricated on the sample 1under test is detected with the TDI sensor 14 in synchronization withmovement of the stage 3 by utilizing position data on the stage 3. Thisposition information is monitored with a length measuring device orposition sensor (not shown in drawing). Brightness information(grayscale image signal) 14 a about the pattern formed on the sample 1is in this way obtained. The grayscale image signal 14 a obtained withthe image sensor 14 is then input to a signal processing circuit 101 tofind pattern defects including foreign matter deposited on the sample 1under test.

[0049] The signal processing circuit 101 is comprised of an A/Dconverter 15, a gray level converter 16, delay memory 17, comparator 18,and central processing unit 19, etc. The A/D converter 15 converts thegrayscale image signal 14 a obtained with the TDI sensor 14 into adigital signal. Here, the A/D converter 15 can also be installed at alocation immediately after (downstream from) the TDI sensor 14, apartfrom the signal processing circuit 101. If necessary, the coherencesuppression optics 8 is internally controlled by an angle controlcircuit 24 from the central processing unit 19.

[0050] The gray level converter 16 consists for example of an 8-bit graylevel converter and performs gray level conversion on the digital imagesignal transferred from the A/D converter 15, as described in JP-A No.320294/1996. The gray level converter 16 performs this conversion usinglogarithmic, exponential and polynomial expressions to correct shadingor uneven brightness on the image caused by laser beam interference withthe thin film formed on the sample 1 under test (such as thin filmsformed on a semiconductor wafer during the wafer process).

[0051] The delay memory 17 stores the image signal transferred from thegray level converter 16, within a period of the scan width of the imagesensor 14, so as to make the delay equal to the one cell or one chip orone shot comprising the sample (semiconductor wafer).

[0052] The comparator 18 compares the image signal transferred from thegray level converter 16 with the image signal obtained through the delaymemory 17, in order to detect mismatches between them as defects. Inother words, the comparator 18 compares the detected image with theimage transferred from the delay memory 17 that was obtained with adelay equal to the cell pitch or to one chip.

[0053] The central processing unit (CPU) 19 creates defect inspectiondata based on inspection results compared by the comparator 18 and alsobased on coordinate positioning data on the sample 1 (semiconductorwafer). This data is obtainable from circuit design information andshould be entered in advance from an input means 20 consisting of akeyboard, storage medium, network, etc. This defect inspection data isstored in the storage unit 21 and can be displayed on a display means 22as needed, or output to an output means 23 for observing the locationsof defects on other review (evaluation) devices.

[0054] The comparator 18 can be configured like as disclosed in JP-A No.212708/1986. The comparator 18 as shown in FIG. 21 consists for exampleof a positioning circuit 181 that aligns the positions of the comparisonimage Ii transferred from the gray level converter 16 and the referenceimage Ir transferred from the delay memory 17, a differential imagedetection circuit 182 that detects a differential image Id between thecomparison image Ii and the reference image Ir that were aligned witheach other by the positioning circuit 181, a mismatch detection circuitthat converts the differential image Id detected by the differentialimage detection circuit 182 into a binary image by setting a thresholdlevel, and a feature extraction circuit 184 that extracts informationabout the area, length and coordinate from the binary output transferredfrom the mismatch detection circuit 183.

[0055] Next, we will explain an embodiment of the ultraviolet laserlight source (ultraviolet laser generator) 5. As stated earlier, shorterwavelengths of illumination light are essential to obtain a higherresolution in pattern defect inspection, and also the sample 1 should beilluminated with higher intensity light to improve the inspection speed.Discharge lamps such as mercury-xenon lamps are widely used asillumination light sources of the prior art. Since these discharge lampsproduce high intensity in the visible region, the line spectra in thevisible region are mainly utilized to obtain higher intensityillumination. Line spectra in the ultraviolet to deep ultraviolet regionare only a few percent of those in the visible region, so a high-powerlamp must be used to obtain the required ultraviolet or deep ultravioletlight intensity.

[0056] When a larger lamp with higher power is used, the optical systemmust be separated from the light source to prevent adverse effects byheat generating from the lamp, but this is not always practical becauseof space limitations. In view of these problems, the invention uses adeep ultraviolet (DUW) laser 5 that emits a DUV laser beam in awavelength range between 100 and to 355 nm.

[0057] Lasers are well known to be coherent light sources (havingcoherence), so when a laser beam illuminates the circuit pattern formedon the sample 1 under test, speckle noise (interference fringe) occurscausing trouble during pattern defect inspection. Because of thisproblem, the invention uses the coherence suppression optics 8 tospatially reduce the coherence of laser beams and minimize specklenoise.

[0058]FIG. 2 is a simplified pictorial drawing of the coherencesuppression optics 8 of the present embodiment. The laser beam L1emitted from the laser light source 5 strikes a mirror 25. This mirror25 is driven by an oscillating motor 26 that oscillates within a smallangle. Since the mirror 25 oscillates with the oscillating motor 26, theoptical axis of the laser beam L1 reflected from the mirror 25 isscanned along the vertical direction. The laser beam L1 reflected fromthe mirror 25 is then guided to a mirror 29 via lenses 27 and 28. Themirror 29 is driven by another oscillating motor 30 that also oscillateswithin a small angle, so that the mirror 29 oscillates as well. When thelaser beam L1 strikes the mirror 29, the optical axis of the reflectedlight is scanned along the horizontal direction. The mirrors 25 and 29are respectively installed at positions conjugate with the focusingposition of the objective lens 11.

[0059]FIG. 3 shows the objective lens 11 viewed from the optical axis.FIG. 4 is a lateral view of the objective lens 11. The laser beamscanned along the vertical direction by the mirror 25 and also along thehorizontal direction by the mirror 29 enters the objective lens 11 as alight flux 31, and is focused on the pupil 12 of the objective lens 11.The light flux 31 focused on the pupil 12 enters a lens 11′ and thenexits from the objective lens 11 as parallel light to illuminate thesample 1. In other words, the sample 1 is subjected to Koehlerillumination.

[0060] By oscillating the mirrors 25 and 29 synchronously with eachother, the light flux 31 annularly scans on the pupil 12 of theobjective lens 11. FIG. 5 shows an example of this operation. Theoscillating motor 26 drives the mirror 25 by using a control curve 32which is usually a sine curve. The oscillating motor 30 on the otherhand, drives the mirror 29 by using a control curve 33 which iscontrolled by shifting the phase 90° with respect to the control curve32. Controlling the mirrors 25 and 29 in this way allows the light flux31 to annularly scan on the pupil 12 of the objective lens 11. As aresult, the sample 1 is illuminated with light whose incident directioncontinuously changes over time. This prevents interference that occursby light input from different directions, that reduces laser beamcoherency.

[0061] In the invention, one annular scan cycle of the light flux on thepupil 12 of the objective lens 11 is synchronized with the chargeintegration time during which each linear image sensor comprising theTDI sensor 14 stores a signal charge upon detecting light. Morespecifically, within one integration time during which each linear imagesensor of the TDI sensor 14 stores a signal charge, the light flux 31annularly scans one or more times on the pupil 12 of the objective lens11. In addition, the annular scan diameter of the light flux 31 on thepupil 12 can be adjusted by changing the amplitude applied to theoscillating motors 26 and 30. For example, when the oscillating motors26 and 30 are driven with amplitude W1 shown in FIG. 5, the annular scandiameter on the pupil 12 will be Fd1 of FIG. 3. When the amplitude issmall like W2 of waveforms 32′ and 33′ in FIG. 5, the annular scandiameter on the pupil 12 will be Fd2 of FIG. 3.

[0062] As explained above, the annular scan diameter of the light flux31 on the pupil 12 can be freely changed by controlling the amplitude ofthe oscillating motors 26 and 30. The annular scan diameter of the lightflux 31 on the pupil 12 may also be changed for each scan when two ormore annular scans are repeated within one integration time during whicheach linear image sensor of the TDI sensor 14 stores a signal charge.

[0063] Operation of a pattern inspection apparatus having the aboveconfigurations is next described in detail.

[0064]FIG. 11 shows how the angle of a light flux illuminating thesample 1 is changed. FIG. 11A shows a light flux 47 that annularly scansthrough a point near the edge of the pupil 12 of the objective lens 11.The main beam of this light flux 47 is at a position Pr1 away from thecenter of the optical axis and is irradiated on the sample at anincident angle of Pr1? by the lens 11′. FIG. 11B shows a light flux 48that annularly scans through a point near the center of the pupil 12 ofthe objective lens 11 by adjusting the amplitude of the oscillatingmotors 26 and 30. This light flux 47 is irradiated on the sample at anincident angle of Pr2? by the lens 11′.

[0065] In this way, the angle of light flux incident on the sample 1changes as the annular scan diameter of the light flux on the pupil 12changes. In other words, as shown in FIG. 6 and FIG. 7, whenilluminating a sample on which an optically transparent thin film isformed, the reflected light intensity from the sample changes as theangle of the light flux illuminating the sample 1 changes. FIG. 12 showschanges in the reflected light intensity when the incident angle ischanged.

[0066] For example, when the sample 1 is illuminated with the light flux47 which is annularly scanning on the outer portion of the pupil 12 asshown in FIG. 11A, changes in the reflected light intensity versus thefilm thickness are plotted as in a curve 49 of FIG. 12. When the sample1 is illuminated with the light flux 48 which is annularly scanning onthe inner portion of the pupil 12 as shown in FIG. 11B, changes in thereflected light intensity versus the film thickness are plotted as in acurve 50 of FIG. 12. The waveform phase of the reflected light intensityshifts as the annular scan radius (Pr, Pr2) of the light flux on thepupil 12 is changed. For example, the reflected light intensity at afilm thickness of T changes greatly depending on the annular scanradius, that is, the incident angle of the light flux illuminating thesample 1. The reflected light intensity will be R2 when the incidentangle is large and R1 when the incident angle is small.

[0067] When the sample is illuminated with a light flux that annularlyscans on the pupil 12 while keeping the scan radius (distance from theoptical axis of the objective lens 11 to the main beam of the light flux31) constant, the reflected light intensity from the sample 1 changes ina sinusoidal waveform as a function of the thickness of the opticallytransparent thin film 35 formed on the sample 1, as shown in FIG. 7. Howthis reflected light intensity changes depends on the scan radius of thelight flux 31 on the pupil 12 as shown in FIG. 12. For example, when thelight flux 31 is irradiated on the thin film 35 having thickness t01 andt11, the change in reflected light intensity from the sample 1 greatlydiffers as shown in FIG. 9 and FIG. 10. When compared to FIG. 9, thechange in the reflected light intensity of FIG. 10 is less affected byfilm thickness variations.

[0068] Making use of this property, the thickness range of the thin film35 formed on the sample 1 is measured beforehand and the scan radius ofthe light flux 31 on the pupil 12 is set so that the change in thereflected light intensity is minimized within the measured thicknessrange. This makes it possible to inspect the pattern formed on thesample 1 while reducing adverse affects caused by the thicknessdistribution of the thin film 35.

[0069] The relation between the incident angle and the light intensityis measured by pre-inspection to obtain the relation between theincident angle and the film thickness, and this data is stored in thestorage means 21. This measurement can be made by giving an angleinstruction to an angle controller 24 from the central processing unit19 shown in FIG. 1.

[0070]FIG. 13 shows results obtained by comparing images between chipson the sample 1. FIG. 13A is a comparison image obtained by subtractingthe output image of the delay memory 17 from the output image of thegray level converter 16. White portions here indicate the difference issmall, while black portions indicate the difference is large. This imagewas obtained by comparing the images without taking the change in lightintensity between chips into account, so portions 52 having a largerdifference are emphasized. If pattern defect inspection is performedunder this condition, the threshold level must be increased to eliminatethe black portions, making it impossible to detect actual defects.

[0071] On the other hand, FIG. 13B shows a comparison image detectedafter optimizing the incident light angle on the sample 1 as shown inFIG. 10. Since the incident angle of the light illuminating the sample 1is set so that the change in the reflected light intensity from thesample 1 is minimized within the thickness range of the thin film 35,changes in the light intensity between chips are also minimized. Thisreduces the difference in the light intensity between the image signalstransferred from the gray level converter 16 and the delay memory 17,and extracts the actual defects 54 from the comparison images.Consequently, pattern defects can be detected with high sensitivity.

[0072] Next, we will explain another embodiment of the coherencesuppression optics 8 using the drawing shown in FIG. 14. In thisembodiment, the laser beam L1 emitted from the laser light source 5 isirradiated onto a homogenizer 55. The homogenizer 55 forms multiplesmall light sources arranged in a matrix pattern at an XY pitch of “a”and “b”, and produces multiple spot light sources from a single lightbeam. The laser beam L1 transformed by the homogenizer 55 into a beamhaving multiple light spots along the cross section passes through alens 56 and strikes a mirror 25. Since the mirror 25 is supported by anoscillating motor 26 that oscillates within a small angle, the opticalaxis of the laser beam L1 scans vertically when reflected from themirror 25, and then enters a mirror 29 via lenses 27 and 28. The mirror29 is supported by an oscillating motor 30 that also oscillates within asmall angle. The optical axis of the laser beam L1 therefore scanshorizontally when reflected from the mirror 29. The mirror 25 and mirror29 are respectively installed at positions conjugate with the focusingposition of the objective lens 11.

[0073] The lenses 56, 27, and 28 are designed and installed so that animage of the homogenizer 55is focused on the pupil of the objective lensto achieve Koehler illumination. FIG. 15 shows how an image of thehomogenizer 55 is focused on the pupil 12 of the objective lens 11. Animage 55′ of the homogenizer 55, which consists of multiple light spotsarranged at an XY pitch of “a” and “b”, is focused on the pupil 12 ofthe objective lens 11. The group of these light spots is rotatedcircularly within the pupil 12 by the oscillating motors 26 and 30. Bychanging the amplitude applied to the oscillating motors 26 and 30, thescan diameter within the pupil 12 can be changed even when using a groupof light spots. This embodiment achieves the same effective results asthe aforementioned embodiment.

[0074] We will further explain another embodiment of the coherencesuppression optics 8 using the drawing shown in FIG. 16. In thisembodiment, the laser beam L1 emitted from the laser light source isirradiated on a fixed mirror 61 and then strikes an angular oscillationmirror 62 via lenses 27 and 28. The angular oscillation mirror 62 issupported by a rotating motor 63. FIG. 17A and FIG. 17B show details ofthe mirror 62. FIG. 17A is a front view as seen from the reflectivesurface of the mirror and FIG. 17B is a side view. FIG. 18 shows themovement track of the oscillation mirror 62 when rotated at a position64 shown in FIG. 17A. The horizontal axis and vertical axis of FIG. 18represent the angle and height, respectively. A sine curve 65corresponds to one rotation of the angular oscillation mirror 62. When alaser beam 67 strikes the angular oscillation mirror 62 at an angle of45°, the angle of the light reflecting from the position 64 changes, sothat a laser beam 68 draws a circular track according to the height ofthe track 65 shown in FIG. 18, which corresponds to one rotation of theangular oscillation mirror 62.

[0075]FIG. 19 shows a cross section of the angular oscillation mirror 62along a line “A-A” shown in FIG. 17. The mirror 62 has a slope 66 in thehorizontal direction. By this slope, the height shown in FIG. 18 changesalong the circular arc direction. As shown in FIG. 16, the laser beamirradiation position on the angular oscillation mirror 62 changes as theangular oscillation mirror 62 and the motor 63 move in the right andleft directions as indicated by arrows in FIG. 16, so a laser beam movesalong a circular arc with a smaller amplitude near the center of themirror 62 and a larger amplitude near the edge. The angular oscillationmirror 62 is installed at a position conjugate with the focusingposition of the objective lens 11. The diameter of the laser beammovement on the pupil of the objective lens can also be changed byadjusting the position of the rotating motor 63. This embodimentachieves the same effective results as the aforementioned embodiment.

[0076] A method for inspecting defects of a circuit pattern formed on asemiconductor wafer is described next by using an inspection apparatusequipped with the devices mentioned in the invention.

[0077] First of all, a wafer 1, that is the sample to be inspected, isplaced on the Z stage 2 and positioned correctly. Next, the Y stage 4holding the wafer 1 moves in the Y-axis direction at a constant speedwhen the stage control circuit 100 receives a signal to drive the stagefrom a stage position sensor (not shown in drawing).

[0078] Meanwhile, the laser beam L1 is emitted from the ultravioletlaser light source 5 and the laser beam diameter is enlarged by the beamexpander 7. The laser beam then enters the coherence suppression optics8 and is output while being scanned by the scanning mirrors 25 and 29 intwo intersecting axial directions. The laser beam emitted from thecoherence suppression optics 8 shifts the optical path at the polarizingbeam splitter 10 and enters the objective lens 11. The objective lens 11condenses the laser beam onto the surface of the wafer 1.

[0079] The laser beam scanned in two intersecting axial directions bythe scanning mirrors 25 and 29 in the coherence suppression optics 8scans along a circle on the pupil plane 12 of the objective lens 11. Thewafer 1 being illuminated at the same time, moves at a constant speed inthe Y axis direction while the incident angle is sequentially changedversus the normal line direction on the wafer 1 at each circular scan.The incident angle of the laser beam L1 striking the wafer 1 isdetermined by the CPU 19 based on the thickness distribution data on theoptically transparent thin film formed on the surface of the wafer 1(This is measured in advance and stored in the storage unit 21.), therelation between the thin film thickness (This is also measured inadvance and stored in the storage unit 21.) and the reflected lightintensity obtained for each incident angle of the laser beam L1 on thewafer 1, and the position information on each stage measured with thestage position sensors (not shown in drawing) for the X stage 3 and Ystage 4. Using these results, the oscillating motors 26 and 30 arecontrolled by the angle control means 24 in order to control the amountof oscillation of the mirrors 25 and 29.

[0080] The wafer 1 is illuminated with a laser beam at an incident angleaccording to the thickness of the optically transparent thin film formedover the surface of the wafer 1. The reflected light from the wafer 1 iscondensed by the objective lens 11 and focused on the TDI sensor 14 by alens 13.

[0081] As mentioned above, the TDI sensor 14 is a time delay integrationimage sensor made up of a number of linear image sensors connected in amultiple stage array. The image signals detected at each stage of thelinear image sensors are sequentially transferred to the linear imagesensor of the next stage and accumulated. This transfer timing issynchronized with the movement of the Y stage 51 that is constantlydetected by the stage position sensor.

[0082] A grayscale image signal 14 a of the wafer 1 acquired with theTDI sensor 14 is converted into a digital signal by the A/D converter15. Uneven brightness or shading on the image caused by interference ofthe laser beam with the thin film formed on the wafer 1 under test iscorrected with the gray level converter 16. The signal processed by thegray level converter 16 is divided into two signals. One is stored inthe delay memory 17 and the other is input to the comparator 18.

[0083] In the comparator 18, the comparison image Ii transferred fromthe gray level converter 16 and the reference image Ir that was detectedin the previous step (adjacent chip or adjacent pattern) and stored inthe delay memory 17 are both input to the positioning circuit 181. Thepositioning circuit 181 finds the positional shift (deviation) betweenthe comparison image Ii and the reference image Ir and corrects thisshift.

[0084] The positioning circuit 181 outputs the comparison image Ii andthe reference image Ir after correcting their mutual positional shift(deviation) and inputs them to the differential image detection circuit182 where a differential image Id between the two images is obtained.The differential image Id obtained here is sent to the mismatchdetection circuit 183 and compared with the preset threshold level.Portions higher than this threshold level are detected as defects. Theinformation about the defects is then sent to the feature extractioncircuit 184.

[0085] The feature extraction circuit 184 extracts information about thearea, length and coordinates of the defects detected by the mismatchdetection circuit 183, and sends the information to the centralprocessing unit (CPU) 19. The central processing unit 19 stores theinformation about the defects in the memory unit 21 and also displays iton the screen of the display means 22. Though not shown in FIG. 1 andFIG. 3, the comparison image Ii that was transferred from the gray levelconverter 16 and whose positional shift was corrected by the positioningcircuit 181, is also input to the central processing unit 19 and storedin the memory unit 21 or displayed as an image containing defects on thescreen of the display means 22 as needed. Information about defectsstored in the memory unit 21 can be transferred via communication linesfrom the output means 23 to other devices such as review (evaluation)devices used to observe a detailed view of the defects.

[0086] Next, the second embodiment of the invention is shown in FIG. 22.Basic components of this second embodiment are identical with those ofFIG. 1 except that the image processing circuit 101 has a slightlydifferent configuration. This embodiment uses a film thickness datainput means that transfers film thickness data 69 to the centralprocessing unit 190 of the image processing circuit 101. We will nowdiscuss the operation of this embodiment. In FIG. 1, inspection isperformed after finding an optimum incident angle by measuring therelation between the film thickness and the reflected light intensity bypre-inspection. In this embodiment, however, the thickness of the filmon the sample 1 is measured in advance with a thickness gauge, etc. andthe film thickness data 69 is input to the central processing unit 190.The central processing unit 190 processes the film thickness data 69 togive an instruction to the angle control circuit 24 according to thethickness of the optically transparent thin film coated over the sample1, so that the sample is illuminated with light at an optimum incidentangle and the image thus acquired is used for pattern inspection. Thissecond embodiment yields the same effective results as the firstembodiment.

[0087] When light intensity changes during inspection of the inventioneven after setting the optimum incident angle of illumination light andthe inspection results fluctuate significantly, a sudden increase infilm thickness variations on the sample has probably occurred. Bychecking the inspection results, sudden variations in film thickness ofthe sample can be monitored to allow process control. FIG. 23 shows aflowchart for process control using pattern inspection of the invention.If abnormal results are found in the test process, that sample is thenremoved and the film thickness re-measured with a thickness gauge toinvestigate the cause. The results are then fed back to the filmdeposition or forming equipment when needed.

[0088] As described above, the invention is capable of reducing adverseeffects from thin film interference, by illuminating the sample with alaser beam at an optimum incident angle according to the thickness ofthe thin film formed on the sample. This invention is therefore capableof canceling out light intensity fluctuations that may occur duringinspection due to thickness variations of the thin film caused by adifference in the circuit pattern density or thickness variations ofthin films among chips that occur depending on sample positions. Patterninspection can therefore be performed with high sensitivity.

[0089] The invention is also effective in process control when suddenfluctuations in the light intensity are detected during inspection.

[0090] The invention may be embodied in other specific forms withoutdeparting from the spirit or essential characteristics thereof. Thepresent embodiment is therefore to be considered in all respects asillustrated and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed is:
 1. A method for detecting pattern defects,comprising: annularly scanning a laser beam emitted from a laser lightsource on a pupil of an objective lens, illuminating said scanned laserbeam onto a sample on which a pattern is formed through said objectivelens, acquiring an optical image of said illuminated sample, andprocessing said acquired image to find defects of said pattern, whereinthe annular scan diameter of said laser beam is determined based on thethickness of said optically transparent thin film.
 2. The method forinspecting pattern defects according to the claim 1, wherein thewavelength of said laser beam illuminating said sample is in theultraviolet region.
 3. The method for inspecting pattern defectsaccording to the claim 1, wherein the wavelength of said laser beamilluminating the sample is from 100 to 355 nanometers.
 4. A method forinspecting pattern defects, comprising: annularly scanning a laser beamemitted from a laser light source on a pupil of an objective lens,illuminating said scanned laser beam on a pattern formed on saidsubstrate and covered with an optically transparent thin film while saidsubstrate is placed on a table which is continuously moving along onedirection, acquiring an optical image of said pattern illuminated withsaid laser beam in synchronization with the annular scan of said laserbeam, and processing the acquired image to find defects of said pattern.5. The method for inspecting pattern defects according to the claim 4,wherein the annular scan diameter of said laser beam is determined basedon the thickness of said optically transparent thin film.
 6. The methodfor inspecting pattern defects according to the claim 4, wherein saidlaser beam illuminating said pattern is in the ultraviolet wavelengthregion.
 7. The method for inspecting pattern defects according to theclaim 4, wherein the laser beam illuminating said pattern has awavelength ranging between 100 and 355 nanometers.
 8. An apparatus forinspecting pattern defects, comprising: a laser light source, a scanningdevice that scans a laser beam emitted from said laser light source, anilluminating means that irradiates said scanned laser beam onto a sampleusing an objective lens, an imaging device that uses an objective lensto acquire an optical image of the pattern of said sample illuminatedwith said laser beam, and an image processing means that find defects ofsaid pattern by processing an image of said sample acquired with saidimaging device, wherein said scanning device annularly scans said laserbeam on the pupil plane of said objective lens and annular scan diameteris determined based on the thickness of said optically transparent thinfilm.
 9. The apparatus for inspecting pattern defects according to theclaim 8, wherein said laser light source emits light in the ultravioletwavelength region to illuminate said pattern.
 10. The apparatus forinspecting pattern defects according to the claim 8, wherein said laserlight source emits light having a wavelength between 100 and 355nanometers to illuminate said pattern.
 11. The apparatus for inspectingpattern defects according to the claim 8, wherein said imaging devicehas a time delay integration sensor to acquire an optical image of saidpattern. coated with an optically transparent thin film
 12. An apparatusfor inspecting pattern defects comprising: a light illuminating opticshaving a laser light source and an objective lens, a table on which asubstrate is placed and capable of moving along at least one direction,a scanning device to annularly scan a laser beam emitted from said laserlight source on a pupil plane of the objective lens of said lightilluminating optics, an imaging device that acquires, in synchronizationwith the annular scan of said laser beam, an optical image of saidsubstrate while said substrate is placed on said table and illuminatedby the annularly scanned laser beam through said light illuminatingoptics, and an image processing means that detects defects of saidpattern by processing an image acquired with said imaging device. 13.The apparatus for inspecting pattern defects according to the claim 12,further comprising a control means which determines the annular scandiameter of said laser beam and sends information about said diameter tosaid scanning device.
 14. The apparatus for inspecting pattern defectsaccording to the claim 13, wherein said control means determines saidannular scan diameter based on the thickness of an optically transparentthin film coated over a pattern formed on said substrate.
 15. Theapparatus for inspecting pattern defects according to the claim 12,wherein said laser light source illuminates said sample with light inthe ultraviolet wavelength region.
 16. The apparatus for inspectingpattern defects according to the claim 12, wherein said laser lightsource illuminates said sample with light having a wavelength between100 and 355 nanometers.
 17. The apparatus for inspecting pattern defectsaccording to the claim 12, wherein said imaging device uses a time delayintegration sensor to acquire an optical image of said pattern.